An optical scanner is a well-known device used to read an original image contained on a graphic or textual document, photograph or the like, and convert the image to electrical signals that a computer can interpret. The scanner provides an interface between the computer and the image which enables the computer to store, modify and reproduce the image. The scanning of text and graphics greatly reduces the time, labor and cost associated with image generation, manipulation and printing.
Relatively inexpensive, hand-held scanners are available which may be connected by a flexible cord to the input port of a laptop or personal computer (PC). The sensing mechanism of the scanner includes a light source and a system for measuring the light reflected from the image to be scanned by means of a photosensitive silicon chip called a charged-coupled device (CCD). Some scanners may use photodiodes instead of CCD's, without altering the principles involved. As the sensing mechanism scans the image, it generates an array of electrical signals which are converted to digital form and handled by the computer's processor. The resulting digital array corresponding to the image of the target surface is known as a bit map or raster image. The computer's software stores the digital data and manipulates it to generate the desired graphic or textual images which may then be displayed, printed or otherwise used.
Such inexpensive scanners typically incorporate light emitting diodes (LED's) as the light source to light the target area of the image to be scanned. Because LED's emit only a single frequency of light (such as red or green), the scanned image will always be monochromatic. The monochromatic image may be generated in scan patterns of black and white (referred to as a line scan), shades of gray (referred to as a gray scale scan) or halftone (referred to as a "dithered" scan, in which black and white dots are grouped together to produce a halftone reproduction simulating shades of gray).
A monochromatic scanner of the type described is not equipped to generate digital data representative of the true colors contained in the target image. This shortcoming is especially undesirable for graphics applications where a multiple color document, photograph or other image is to be scanned. While color scanners are available, they are much more expensive than monochromatic scanners.